This proposal outlines studies of the structure of CD4; important as an integral cell surface glycoprotein in T cell function, but of recent notoriety as a specific cell-surface receptor to the human immunodeficiency virus (HIV). The investigational approach is through the analysis of individual domains of the CD4 protein. Using a truncated genetic construct, the first (N-terminal) domain, implicated in the major extracellular interactions of CD4, will be produced in bacterial expression systems. The patterns of binding to various anti-CD4 monoclonal antibodies will provide evidence for the ability of an isolated domain of an intact protein to present the same epitopes to those ligands. Other ligands will be assessed for their ability to bind the independent domains, including class 11 MHC antigens and gpl2O, the envelope glycoprotein of the HIV virus which is the specific binding site for CD4. The aim is to use these molecular methods to probe ligand interactions. This will include the use of small inhibitor molecules developed to inhibit the binding of HIV to CD4. These molecules, whose structures are based on homology to CD4, inhibit the infectivity of HIV for target cells. Initial studies suggest that this is due to the binding of the HIV envelope glycoprotein gpl2O with these molecules. Additional structural studies employing mutagenesis of CD4 and its individual domains to detect changes in ligand binding will be performed. Labelled compounds will be used as probes of the mechanism of interaction of these molecules with gpl2O and HIV. These studies will permit refinement of these molecules and have potential use as therapeutic interventions. The candidate has extensive support for her professional interests additionally through her association with the Division of Hematology at the Brigham & Women's Hospital as an associate physician, maintaining clinical skills and a valued academic liaison. The research support extends beyond the immediate laboratory environment, with collaborations in the Department of Infectious Diseases as well as the Department of Chemistry at Harvard University. These interactions within the network of the Harvard Medical Community provide a unique advantage for the pursuit of scientific endeavors.